1. Field of the Invention
This invention relates to an image heating apparatus suitable for use as a heat fixing device carried on an image forming apparatus such as a copying machine or a printer, and particularly to an image heating apparatus having a heater and a flexible rotatable member rotated while contacting with the heater, and for heating an image through the flexible rotatable member.
2. Description of Related Art
Description will hereinafter be made of an example in which a conventional heating apparatus is applied as an image heating apparatus (fixing apparatus) provided in an image forming apparatus such as a copying machine or a printer for heating and fixing a toner image on a recording material.
In image forming apparatuses, a heating apparatus of a heat roller type has been widely used as a fixing apparatus for heating and fixing an unfixed image (toner image) of image information formed and borne on a recording material (such as a transferring material sheet, an electrofax sheet, electrostatic recording paper, an OHP sheet, printing paper or format paper, in a suitable image forming process means portion such as an electrophotographic process, an electrostatic recording process or a magnetic recording process by a transferring method or a direct method as a permanent fixed image on the surface of the recording material.
In recent times, a heating apparatus of a film heating type has been proposed and put into practical use from the viewpoints of quick start and energy saving.
This heating apparatus of the film heating type, as shown in FIG. 10 of the accompanying drawings, is such that film (rotatable member) 52 generally including therein a ceramic heater (hereinafter referred to also as the heater) 51 as a heating member and a guide member 54, and a pressure roller 53 as a discrete rotatable member brought into pressure contact with the film 52 are supported by a supporting member, not shown, and the two rotatable members 52 and 53 are pressed by pressing means 55 to thereby form a pressure contact nip part N.
The film 52 is driven to rotate around the guide member 54 with the inner surface thereof sliding in close contact with the heating surface of the heater 51 at the pressure contact nip part N by the pressure roller 53 being rotatively driven. In this case, in order to reduce the sliding friction resistance between the inner surface of the film and the heater or the guide member to thereby smooth the rotation of the film 52, a lubricant such as heat-resistant grease is interposed between the heating surface of the heater 51 and the inner surface of the film.
That is, the heat-resistant film 52 (hereinafter referred to also as the fixing film, the fixing belt or the film) is nipped between the heater 51 and the pressure roller 53 as a pressure member to thereby form the pressure contact nip part (a heating nip part or a fixing nip part), and a recording material P as a material to be heated on which an unfixed toner image to be fixed is formed and borne is introduced into between the fixing film 52 and the pressure roller 53 at the pressure contact nip part N and is nipped and transported with the fixing film 52, whereby at the pressure contact nip part N, the heat of the ceramic heater 51 is given to the recording material P through the fixing film 52, and the unfixed toner image is heat pressure-fixed on the surface of the recording material by the pressure force of the pressure contact nip part N.
The image heating apparatus applied as the above-described fixing apparatus can also be used, for example, as an apparatus for heating a recording material bearing an image thereon to thereby improve a surface property such as gloss, an apparatus for tentatively fixing an image, etc.
This image heating apparatus of the film heating type can constitute an apparatus of an on-demand type by the use of a ceramic heater and a member of low heat capacity as the fixing film, and has the merits that only during the execution of image forming of the image forming apparatus, the ceramic heater can be electrically energized and brought into a state in which it generates heat to a predetermined fixing temperature, and that the waiting time from the switching-on of the power source of the image forming apparatus till a state in which image forming can be executed is short (a quick starting property) and the electric power consumption during standby becomes greatly small (electric power saving).
In the image heating apparatus of the film heating type, methods of driving cylindrical or endless fixing film as a rotatable member include a method of rotating the fixing film brought into pressure contact by a film guide member (film supporting member) for guiding the inner peripheral surface of the fixing film and a pressure roller by the rotative driving of the pressure roller (a pressure roller driving method), a method of rotating a pressure roller by the driving of endless fixing film passed over a driving roller and a tension roller (a fixing film driving method), etc.
However, the above-described heating apparatus (fixing apparatus) of the film heating type has suffered from the following problems.
There has been the tendency of rotational torque increasing with an increase (endurance) in the number of printed sheets. That is, with an increase (endurance) in the number of printed sheets, the film or the heating member is gradually shaved by sliding friction or the like, and the lubricant is deteriorated under the influence of the shavings thereof or the like and circulation becomes bad. As the result, the mutual frictional forces of the film and the heating member are increased and thus, rotational torque is increased.
Accordingly, when an attempt is made to make the apparatus higher in speed or longer in life, there occurs the inconvenience that for example, a driving motor loses synchronism due to the increase in the torque or paper becomes untransported (fixing slip jam) due to the film being faultily driven. Also, in an extreme case, there has occurred the inconvenience that the film is damaged.
In recent years, further speedup, electric power saving, lower costs and downsizing have been required of image forming apparatuses such as copying machines and printers, and the toner fixing property onto the recording material must be secured while speedup is accomplished. Therefore, when a pressure force is increased, the driving motor loses synchronism due to an increase in rotational torque, or an inconvenience such as fixing jam occurs.
Or it has been practised to increase the width of the heating member in order to increase the width of the fixing nip (secure the toner fixing property), but an increase in the width of the heating member means an increase in the cost of materials or the larger size of the heating member, and such members as the film and the supporting member become large, and a unit itself becomes bulky.
Further, it as shown in FIG. 9 of the accompanying drawings, the width of the heating member is made small and the width of the nip is secured, in a case where the heater protrudes from the film guide member, the film 52 when it slides strongly strikes against the edge portion of the heating member 51 and therefore, the friction between the film 52 and the heating member 51 becomes great, and rotational torque is increased. Further, when the recording material P is supplied, it strongly strikes against the edge portion of the heating member 51, whereby a transport load suddenly rises and the pressure roller 53 becomes unable to transport the recording material P, that is, a so-called slip phenomenon occurs. Of course, if the width of the nip is made small so that the recording paper may not strike against the edge portion, it becomes impossible to secure the fixing property. Or when the heater is retracted from the film guide member, the heat of the heater is not property transferred to the film, and this gives rise to the problem that the fixing property cannot be secured.